Flash

The Flash memory can be divided into three sections: Boot Loader (BOOT), Application Code (APPCODE) and Application Data (APPDATA). The main difference between these sections are access privileges:

The code in the BOOT section can write to APPCODE and APPDATA
The code in APPCODE can write to APPDATA
The code in APPDATA cannot write to Flash or EEPROM

Figure 1 shows how the Flash sections are ordered in the Flash.

Figure 1. Flash Sections

FLASHSTART is at 0x0000 when accessed as program memory, and mapped with the following offsets when accessed via data memory:

megaAVR 0-series: 0x4000
tinyAVR 0- and 1-series: 0x8000

The address mapping is needed for access using normal Load/Store Indirect instructions. In the device header file, the offset is defined as MAPPED_PROGMEM_START, so if accessing Flash address 0x100 via data memory, the Address Pointer can be defined as in the example below:

uint8_t *flash_pointer = (uint8_t *) 0x100 + MAPPED_PROGMEM_START;

The size of the Flash sections can be configured through the BOOTEND and APPEND fuses in steps of 256 bytes (128 words). The following table shows how these fuses configure the sections.

Table 1. Setting Up Flash Sections
BOOTEND	APPEND	BOOT Section	APPCODE Section	APPDATA Section
0	0	0 to FLASHEND	-	-
> 0	0	0 to 256*BOOTEND	256*BOOTEND to FLASHEND	-
> 0	== BOOTEND	0 to 256*BOOTEND	-	256*BOOTEND to FLASHEND
> 0	> BOOTEND	0 to 256*BOOTEND	256BOOTEND to 256APPEND	256*APPEND to FLASHEND

A good way of making sure these fuses are set up as expected on a device is to use the FUSES macro in the bootloader code project. It can be found in fuse.h, which is included by io.h:

#include <avr/io.h>
                
FUSES = {
	.OSCCFG = FREQSEL_20MHZ_gc,
	.SYSCFG0 = CRCSRC_NOCRC_gc | RSTPINCFG_UPDI_gc,
	.SYSCFG1 = SUT_64MS_gc,
	.APPEND = 0x00, // Application data section disabled
	.BOOTEND = 0x02 // Boot section size = 0x02 * 256 bytes = 512 bytes 
};

This will compile the fuse settings into the elf-file for the bootloader, and if this is used to program the device instead of the hex-file, the fuse settings will be programmed at the same time as the Flash.

Note: All fuse bytes in the struct must be configured, not only BOOTEND and APPEND. This is because an omitted fuse byte will be set to 0x00 and may cause an unwanted configuration.

The device fuses can also be configured directly from Atmel^® Studio 7.0, using Device Programming (Ctrl+Shift+P) - Fuses, as shown in Figure 2.

Figure 2. Configure BOOTEND and APPEND fuses, Atmel Studio 7.0